KR101353634B1 - Low alloy cold rolled steel sheet having excellent weldability and strength and method for manufacturing the same - Google Patents

Abstract

The present invention relates to a low alloy cold rolled steel sheet excellent in weldability and strength, and to a low alloy cold rolled steel sheet excellent in weldability and strength that can be used in building materials, home appliances, and automobiles.
One embodiment of the present invention by weight, C: 0.01 ~ 0.12%, Si: 0.1 ~ 2.0%, Mn: 0.5 ~ 2.5%, P: 0.001 ~ 0.10%, S: 0.020% or less, Sol.Al: 0.02 0.30%, N: 0.020% or less, Cr: 0.1-1.2%, B: 0.0010-0.0080%, balance Fe and other unavoidable impurities, including Ti: 0.003-0.08% and Nb: 0.003-0.08% It further comprises one or two of the following, satisfies the conditions of the following relations 1 and 2, the microstructure is excellent in weldability and strength, including more than 95 area% ferrite and less than 5% bainite and martensite An alloy cold rolled steel sheet is provided.
[Relation 1]
C + Mn / 6 + Cr / 5 ≤ 0.55
[Relationship 2]
C + Mn / 20 + Si / 30 + 2P + 4S ≤0.27
According to an aspect of the present invention, it is possible to provide a low alloy cold rolled steel sheet excellent in yield strength, yield ratio, elongation and bending workability as well as weldability and tensile strength, and a method of manufacturing the same.

Description

LOW ALLOY COLD ROLLED STEEL SHEET HAVING EXCELLENT WELDABILITY AND STRENGTH AND METHOD FOR MANUFACTURING THE SAME}

The present invention relates to a low alloy cold rolled steel sheet excellent in weldability and strength, and to a low alloy cold rolled steel sheet excellent in weldability and strength that can be used in building materials, home appliances, and automobiles.

Recently, steel sheets for automobiles are required to have higher strength steel sheets for fuel efficiency improvement or durability improvement due to various environmental regulations and energy use regulations. In particular, recently, as the impact stability regulations of automobiles have spread, high-strength steel having excellent yield strength has been adopted for structural members such as members, seat rails, and pillars to improve impact resistance of the vehicle body. The structural member has a characteristic that the higher the yield strength than the tensile strength, that is, the higher the yield ratio (tensile strength / yield strength), the better the impact energy absorption capacity. However, in general, as the strength of the steel sheet increases, the elongation decreases, so that a problem arises in that moldability deteriorates. Therefore, there is a demand for development of a material that can compensate for this.

Typically, the method of reinforcing steel includes solid solution strengthening, precipitation strengthening, strengthening by grain refinement, transformation strengthening, and the like. However, the reinforcement by solid solution strengthening and grain refinement of the method has a disadvantage that it is very difficult to produce high strength steel with a tensile strength of 490MPa or more.

On the other hand, precipitation-reinforced high-strength steels are formed by adding carbon and nitride forming elements such as Cu, Nb, Ti, and V to precipitate carbon and nitride to reinforce steel sheets or to refine grains by suppressing grain growth by fine precipitates. To secure the technology. The above technique has the advantage of easily obtaining a high strength compared to a low manufacturing cost, but the recrystallization temperature is rapidly increased by the fine precipitate, there is a disadvantage that a high temperature annealing must be performed to ensure ductility sufficient to recrystallize. In addition, the precipitation-reinforced steel which precipitates and strengthens carbon and nitride on a ferrite base has a problem in that it is difficult to obtain high-strength steel of 600 MPa or more.

Representative technologies for the precipitation-reinforced high-strength steel include Japanese Laid-Open Patent Publication No. 56-084422, Japanese Laid-Open Patent Publication No. 04-221015 and Korean Laid-Open Patent Publication No. 2006-0072701. -084422 is a technology to secure the strength by forming a very fine precipitate by setting the coiling temperature to 450 ℃ or less, but the coiling temperature is too low to secure the yield strength of more than 750MPa, as well as causing hot-rolled sheet shape defects However, there is a problem in that an overload phenomenon occurs during cold rolling due to an increase in residual stress around the precipitate.

Japanese Patent Application Laid-Open No. 04-221015 has a problem in that the winding temperature is set to 400 ° C. or less, which not only has the above-mentioned problems but also has a low yield ratio due to the formation of bainite or martensite structure. Patent Publication No. 2006-0072701 has a drawback in that a large amount of alloying elements are added, resulting in high manufacturing costs and an overload phenomenon during cold rolling of more than 50% due to high hot rolled strength.

On the other hand, the transformation hardened high-strength steel is a ferritic-martensitic dual phase steel in which hard martensite is included in the ferritic base, and a transformation induced plasticity (TRIP) steel or a ferritic material using transformation organic plasticity of retained austenite. Although various types of CP (Complexed Phase) steels composed of hard bainite or martensite structure have been developed, alloying elements such as Mn and Si in a large amount to form a hard phase or austenite for causing transformation hardening There is a problem that must be added, such a hardenable element has a disadvantage in that the weldability, and the annealing Mn, Si-based oxide is concentrated on the surface, the plating properties are lowered.

Representative technologies for the transformation steel is Japanese Unexamined Patent Publication No. 06-145892, Japanese Patent No. 2660644, Japanese Patent No. 2704350, Japanese Patent No. 3317303, etc. It is a technology to secure the ductility or formability by controlling the amount of residual austenite or by removing other microstructures. However, the above techniques have a low yield strength to improve ductility, which has the disadvantage that the impact resistance is lowered, and since only the ductility is taken into consideration, the bending workability, hole expandability, or weldability, etc. required for the actual machining of parts Not enough consideration was given.

On the other hand, an essential requirement for a high strength steel sheet having a yield strength of 750 MPa or more to be used as a structural member or a collision member is weldability. Structural steel or stiffeners serve to protect passengers by absorbing impact energy during a collision. If the strength of the weld is not sufficient, the structural steel or stiffener may break during collision to obtain sufficient impact absorption energy. Therefore, there is a demand for development of a high strength steel sheet in consideration of such weldability, and Japanese Patent Laid-Open Publication No. 2003-193194 has a technique for solving this problem. However, the above technique has a problem in that it does not satisfy the weldability required in the actual market.

Another technique is Japanese Patent Application Laid-Open No. 2005-105367, which proposes a technique for securing high yield and weldability and ductility for steel having a tensile strength of 780 MPa or more. However, when the above technique is applied to the actual process, shape control is difficult due to the high strength of the intermediate material hot rolled steel sheet, cold rolling property is greatly reduced by increasing the rolling load, and the quench heat treatment condition must be applied during annealing heat treatment. Therefore, there is a problem that the operability such as shape control of the annealing material and the occurrence of surface defects are greatly reduced.

One aspect of the present invention is to provide a low-alloy cold rolled steel sheet and a method of manufacturing the improved alloy, such as yield strength, yield ratio, elongation and bending workability with excellent weldability and tensile strength without a small amount of alloying elements such as water cooling.

One embodiment of the present invention by weight, C: 0.01 ~ 0.12%, Si: 0.1 ~ 2.0%, Mn: 0.5 ~ 2.5%, P: 0.001 ~ 0.10%, S: 0.020% or less, Sol.Al: 0.02 0.30%, N: 0.020% or less, Cr: 0.1-1.2%, B: 0.0010-0.0080%, balance Fe and other unavoidable impurities,

In addition, one or two of Ti: 0.003 to 0.08% and Nb: 0.003 to 0.08% are further included.

Satisfy the conditions of the following relations 1 and 2,

The microstructure provides a low welded cold-rolled steel sheet with excellent weldability and strength, including ferrites of more than 95 area% and less than 5% bainite and martensite.

[Relationship 1]

C + Mn / 6 + Cr / 5 ≤ 0.55

[Relation 2]

C + Mn / 20 + Si / 30 + 2P + 4S ≤0.27

A plating layer may be formed on the surface of the cold rolled steel sheet.

One embodiment of the present invention by weight, C: 0.01 ~ 0.12%, Si: 0.1 ~ 2.0%, Mn: 0.5 ~ 2.5%, P: 0.001 ~ 0.10%, S: 0.020% or less, Sol.Al: 0.02 0.30%, N: 0.020% or less, Cr: 0.1-1.2%, B: 0.0010-0.0080%, balance Fe and other unavoidable impurities, including Ti: 0.003-0.08% and Nb: 0.003-0.08% Re-heating the slab further comprising one or two of the following, satisfying the conditions of the following relations 1 and 2 to 1100 ~ 1300 ℃; Finishing hot rolling the reheated slab above Ar3 transformation point to obtain a hot rolled steel sheet; Cooling the hot rolled steel sheet at a rate of 20 to 80 ° C./s; Winding the cooled hot rolled steel sheet in a range of Coiling Temperature (CT) represented by the following Equation 3; Cold rolling the hot rolled steel sheet at a rolling reduction rate of 10 to 34% to obtain a cold rolled steel sheet; And it provides a method for producing a low alloy cold rolled steel sheet excellent in weldability and strength, including annealing the cold rolled steel sheet at 500 ~ 650 ℃.

[Relationship 1]

C + Mn / 6 + Cr / 5 ≤ 0.55

[Relation 2]

C + Mn / 20 + Si / 30 + 2P + 4S ≤0.27

[Relation 3]

555-440C-14Si-26Mn-11Cr ≤ CT ≤ 600-58C-35Mn-34Cr

Preferably, the microstructure of the hot rolled steel sheet includes ferrite and lower bainite.

The annealing is preferably performed for 3 to 100 hours, and after the annealing step, the annealing can be performed at a rate of 30 ° C / s or less.

In addition, the method may further include forming a plating layer on the cold rolled steel sheet.

According to an aspect of the present invention, it is possible to provide a low alloy cold rolled steel sheet excellent in yield strength, yield ratio, elongation and bending workability as well as weldability and tensile strength, and a method of manufacturing the same.

Hereinafter, as an aspect of the present invention, a low alloy cold rolled steel sheet excellent in weldability and strength will be described.

The inventors of the present invention, in the production of high strength steel sheet having a yield strength of 750 MPa or more, all the weldability, tensile strength, yield strength, yield ratio, elongation, bending workability, etc., even without a small amount of alloying components, without performing quenching such as water cooling after annealing As a result of research on a method for producing a good steel sheet, it is recognized that if the alloy composition, range and microstructure are properly controlled, a steel sheet excellent in all of the above characteristics can be manufactured. Particularly, in order to achieve the microstructure to be obtained by the present invention, the present invention is based on the fact that the controlled cooling at the time of hot rolling, the low pressure at the time of cold rolling, and the annealing are effective for producing the cold rolled steel sheet of the present invention. To complete.

Hereinafter, the composition of the present invention will be described in detail.

Carbon (C): 0.01 to 0.12% by weight

C is an important element in depositing carbide in combination with a carbide forming element or securing strength by strengthening ferrite solid solution, and the C is preferably added in an amount of 0.01% or more. However, since the weldability may be lowered when it exceeds 0.12%, the range of C is preferably 0.01 to 0.12% by weight.

Silicon (Si): 0.1-2.0 wt%

Si suppresses the formation of a group of pearlite by improving ferrite strength and inhibiting carbide precipitation by solid solution strengthening, but for this effect, Si is preferably added at least 0.1%. However, when the Si is added in a large amount, the rolling property may be greatly deteriorated, and not only may cause surface scale defects in relation to the surface properties, but also lower the surface properties of the plated steel sheet and lower the chemical conversion treatment. The content of Si is preferably made 2.0% or less.

Mn: 0.5-2.5 wt%

Mn is an element having a very high solid solution effect and at the same time promotes the formation of a composite structure composed of ferrite and martensite. For this effect, Mn is preferably added at least 0.5%. However, when the Mn exceeds 2.5%, there is a high possibility that problems such as weldability and hot rolling property occur. Therefore, the Mn content is preferably in the range of 0.5 to 2.5% by weight.

Phosphorus (P): 0.001 to 0.10 wt%

P is an element exhibiting an effect of strengthening the steel sheet, and in order to improve the strength, P is preferably added at 0.001% or more. If less than 0.001%, strength is not easily secured and may cause a problem in manufacturing cost. On the contrary, since excessive addition may deteriorate press formability and cause brittleness of steel, P is preferably added at 0.10% or less.

Sulfur (S): 0.020 wt% or less

S is an impurity element in steel and is an element that hinders ductility and weldability of a steel sheet. When the content of S exceeds 0.02%, there is a problem that the possibility of inhibiting the ductility and weldability of the steel sheet is high.

Soluble Aluminum (Sol.Al): 0.02 ~ 0.30 wt%

Sol.Al prevents the formation of nonmetallic inclusions during solidification by removing oxygen present in the steel and increases the hardenability by distributing carbon in the ferrite as austenite like Si. For this effect, it is preferable to add more than 0.02% of Sol.Al, but when added excessively, the effect is not only saturated, but the manufacturing cost can be increased, so that the Sol.Al is added below 0.30%. It is preferable to be.

Nitrogen (N): 0.020 wt% or less

N is an impurity element, which is inferior in continuous castability and increases the production cost. Therefore, the steel sheet of the present invention is preferably not included as much as possible, but considering the amount inevitably contained in the manufacturing process, it is preferable to limit the upper limit to 0.02% or less.

Chromium (Cr): 0.1-1.2 wt%

Cr is a hardenable element that prevents surface decarburization and also promotes the formation of low temperature transformation tissue upon cooling. Precipitates due to precipitate formation elements such as Ti and Nb are not easy to reprecipitate after melting due to rapid high temperature and rapid cooling at the time of welding, which causes hardness deterioration of the weld heat affected zone. By promoting it, it has the effect of compensating for it. In addition, Cr is advantageous in improving the hardenability of the steel and securing strength. According to one embodiment of the present invention, Cr plays an important role as a bainite formation promoting element. However, when the Cr is added to less than 0.1%, the effect is almost not, on the contrary, when a large amount is added, the effect is not only saturated but also economical, the content of Cr is preferably 1.2% or less. Do.

Boron (B): 0.0010 to 0.0080 wt%

B is a component that delays the transformation of austenite into pearlite during cooling during annealing, and is an element that inhibits ferrite formation and promotes the formation of bainite. However, when the content of B is less than 0.001%, it is difficult to obtain the above effect. On the contrary, when the content of B is more than 0.008%, grain boundary embrittlement due to grain boundary segregation and excessive concentration of B on the surface may cause plating adhesion deterioration.

In one embodiment of the present invention, it is preferable to further include one or two of Ti: 0.003-0.08% by weight and Nb: 0.003-0.08% by weight in the steel composition as described above, thereby increasing strength and grain size The refinement effect can be improved. However, when the Ti and Nb is added less than 0.003%, it is difficult to secure the effect, and when the Ti and Nb exceeds 0.08%, there is a problem in that ductility may be greatly reduced due to an increase in manufacturing cost and formation of excessive precipitates.

The steel sheet according to an embodiment of the present invention preferably satisfies the following Formulas 1 and 2, while satisfying the composition and range of the alloy component.

[Relationship 1]

C + Mn / 6 + Cr / 5 ≤ 0.55

C, Mn, Cr in the steel serves to increase the carbon equivalent during arc welding, the higher the carbon equivalent is deteriorated weldability. That is, elements such as C, Mn, Cr, and the like are closely related to the improvement in weldability, and the present inventors have created the relational expression 1 expressed as described above through research to improve the weldability by controlling the components. Denotes a component relationship in which arc weldability quality can be secured, and weldability can be improved by satisfying the above relational expression. However, when the value of the relationship 1 is not satisfied, there is a problem that the possibility of welding failure may increase.

[Relation 2]

C + Mn / 20 + Si / 30 + 2P + 4S ≤0.27

The relational formula 2 is created by the present inventors through research, and represents a component relationship capable of securing resistance spot weldability. The elements such as C, Mn, Si, P, S also serves to increase the carbon equivalent, and as is well known, the higher the carbon equivalent, the lower the weldability, so appropriate control of the element is necessary. The present inventors improve the weldability by setting the steel plate satisfying this by setting the relation 2 as described above, if the value of the relation 2 is not satisfied, there is a problem that the possibility of welding failure can be increased.

On the other hand, the microstructure of the cold rolled steel sheet according to an embodiment of the present invention preferably contains more than 95 area% of ferrite, in this way, by securing a large amount of ferrite, while improving the tensile strength and yield strength and high yield ratio Can be achieved, and also bendability can be improved. According to one embodiment of the present invention, it is preferable that the microstructure of the steel sheet is made entirely of a ferrite structure, that is, only a ferrite structure, but the present invention obtains even if it contains less than 5% of the two-phase structure that can be inevitably formed. It can secure its characteristics. Here, the biphasic tissue refers to low temperature transformation tissue such as bainite tissue or martensite.

EMBODIMENT OF THE INVENTION Hereinafter, one Example of the manufacturing method for obtaining the steel plate of this invention is demonstrated.

After satisfying the alloy composition and component range as described above, and preparing a steel slab that satisfies relations 1 and 2, it is preferable to reheat the slab to 1100 to 1300 ° C. If the steel slab heating temperature is less than 1100 ℃, the homogenization of the playing tissue is not enough and the problem of difficult to secure the temperature during the finish rolling, and if it exceeds 1300 ℃ crystal grain size increases and surface oxidation occurs to reduce the strength or surface characteristics Since this inferior problem may occur, the steel slab heating temperature is preferably 1100 to 1300 ° C.

Then, it is preferable to finish hot rolling the reheated slab above the Ar3 transformation point. The reason why the finishing hot rolling temperature is controlled above the Ar3 transformation point is to prevent the two-phase rolling from occurring. When the two-phase rolling is performed, workability deterioration due to the nonuniformity of the microstructure occurs due to the formation of the mixed structure. . In addition, when the finish hot rolling below the Ar3 transformation point, it becomes difficult to control the microstructure of the hot rolled steel sheet. In the present invention, the finishing hot rolling temperature is not particularly limited, but after reheating, hot rolling is performed, and thus the finishing hot rolling temperature cannot exceed 1300 ° C. Therefore, the finish hot rolling temperature range is preferably Ar3 ~ 1300 ℃.

After the hot rolling is finished as described above, cooling and winding are performed. At this time, the cooling rate and the odor temperature are the main factors proposed by the present invention, and the yield strength, the elongation ratio, the yield ratio, In order to impart bending workability, it is an essential condition to be satisfied.

As described above, it is preferable to cool the hot rolled steel sheet obtained through hot rolling at a rate of 20 to 80 ° C / s. As described above, by performing cooling after hot rolling at 20 ° C./s or more, it is possible to prevent the ferrite transformation from occurring in a large amount and to suppress the reduction of the strength of the hot rolled steel sheet too much. Mechanical properties can be secured. On the other hand, when the cooling rate exceeds 80 ° C / s, the strength of the hot rolled steel sheet is excessively increased, since the cold rolling may be inhibited. Therefore, the cooling rate is preferably in the range of 20 ~ 80 ℃ / s.

After the cooling step, it is preferable to perform frying in the coiling temperature (Coiling Temperature, CT) range represented by the following relational formula (3). This is to ensure that the microstructure of the steel sheet contains a certain amount of bainite, and through this, the microstructure of the hot rolled steel sheet may be made of ferrite and lower bainite. As such, the microstructure of the hot rolled steel sheet is made of ferrite and lower bainite, thereby ensuring an appropriate level of hot rolled steel sheet strength.

[Relation 3]

555-440C-14Si-26Mn-11Cr ≤ CT ≤ 600-58C-35Mn-34Cr

Thereafter, it is preferable to obtain the cold rolled steel sheet by cold rolling the hot rolled steel sheet at a reduction ratio of 10 to 34%, and to increase the strength due to work hardening through the above low pressure, thereby increasing the strength above a target level. It can be secured. In addition, by setting the reduction ratio as described above, recrystallization of the crystal grains can be delayed, so that an annealing step, which is a subsequent step, can be performed under the component conditions of the present invention. Then, on the other hand, the reduction ratio of 10 to 34% is a level under light pressure, not more than 40% commonly used in the art. That is, the high strength steel of yield strength of 750 MPa or more proposed by the present invention is difficult to control the shape due to the high strength of the hot rolled steel sheet, which is an intermediate material, and the cold rolling property can be greatly reduced by increasing the rolling load. By controlling at a low level, the cold rolling load can be reduced to solve the above problems, and in addition, the productivity can be exerted. On the other hand, when the reduction ratio is less than 10%, there is a problem that the recrystallization driving force is weakened to obtain good recrystallized grains, and when the reduction ratio exceeds 34%, the material is softened and it is difficult to secure a target strength. .

Subsequently, it is preferable to anneal the cold rolled steel sheet, thereby decomposing the bainite structure contained in the hot rolled structure. This allows the microstructure to be transformed into ferrite, thereby exhibiting a ductility enhancement effect. However, the bainite may be transformed into martensite during decomposition or may not be completely decomposed, and thus the steel of the present invention may include less than 5% of bainite or martensite structure. It is preferable that the said annealing is performed at 500-650 degreeC. When the annealing temperature is less than 500 ° C., there is a risk of inferior elongation due to unrecrystallization, and when it exceeds 650 ° C., the strength may deteriorate due to excessive recrystallization and formation of large grains. In addition, it is possible to reduce the surface quality, such as surface defects caused by the surface annealing concentration, such as poor shape due to high temperature annealing operation.

The annealing time during the annealing greatly affects the strength and the elongation. To obtain a sufficient elongation while having a yield strength of 780 MPa or more, the cold-rolled sheet must be properly recrystallized and maintained for at least 3 hours. However, when more than 100 hours, the recrystallization is excessive, the strength may decrease due to the formation of coarse grains, the production cost is increased by lowering the productivity and energy consumption required for maintaining high temperature, the annealing time It is preferable to have a range of 3 to 100 hours.

By satisfying the above process conditions, even after being cooled at a relatively slow rate of less than 30 ℃ / s, the steel can secure a high strength of 750MPa or more yield strength. In other words, the alloy component proposed in the present invention and the steels satisfying the equations (1) and (2) have a low hardenability and cause a relatively soft phase transformation compared to martensite during cooling, such that water cooling reaches a cooling rate of 80 to 100 ° C / s. Without equipment, it is difficult to produce high strength steel having a yield strength of 750 MPa or more. The above-described manufacturing conditions, that is, the low rolling and annealing processes during hot rolling control cooling and cold rolling, do not quench the steel material and perform relatively slow speeds. High strength can be secured even by cooling.

On the other hand, the present invention may further include the step of forming a plating layer on the cold rolled steel sheet manufactured as described above, through which, it can be applied to automotive steel sheets, building materials and home appliances. The plating forming method may be electroplating, hot dip plating, and the like, and may be alloyed after the plating, and a person having ordinary knowledge in the art may select and use a type of plating layer according to an applied field. .

Hereinafter, the present invention will be described in more detail with reference to Examples. It should be noted that the following examples are merely illustrative for explaining the present invention in more detail, and do not limit the scope of the present invention.

(Example)

The steel material having the alloy composition as shown in Table 1 was reheated at 1300 ° C., and then hot rolled at a finish hot rolling temperature of 880 ° C. Subsequently, a cold rolled steel sheet was manufactured using the manufacturing conditions shown in Table 2 below.

division Chemical composition (% by weight) C Si Mn P S Al Cr Ti Nb B N Relationship 1 Relation 2 Inventive Steel 1 0.04 0.99 1.65 0.011 0.003 0.035 0.72 0.06 0.033 0.001 0.006 0.46 0.190 Invention river 2 0.08 1.47 1.51 0.019 0.005 0.271 0.65 0.01 0.03 0.001 0.007 0.46 0.263 Invention steel 3 0.07 1.08 1.77 0.015 0.002 0.031 0.85 0.06 0.028 0.005 0.007 0.54 0.233 Comparative River 1 0.14 1.51 2.01 0.007 0.003 0.028 - - 0.032 - 0.06 0.48 0.317 Comparative River 2 0.14 1.49 2.00 0.01 0.004 0.029 - - - - 0.008 0.47 0.326 Comparative Steel 3 0.08 1.02 2.10 0.01 0.009 0.03 0.79 0.031 0.034 - 0.009 0.59 0.275 [Relationship 1] C + Mn / 6 + Cr / 5 <0.55
[Relationship 2] C + Mn / 20 + Si / 30 + 2P + 4S ≤0.27

division Steel grade After hot rolling
Cooling rate
(° C / s) Winding temperature upper limit (℃) Winding temperature lower limit (℃)
Temperature
(℃) Cold
Reduction rate
(%) Annealing temperature
(℃) Annealing time
(Hr) Inventory 1 Inventive Steel 1 40 515 473 480 10 500 5 Inventive Example 2 Inventive Steel 1 40 515 473 480 30 600 30 Inventory 3 Invention river 2 40 520 453 500 20 600 96 Honorable 4 Invention steel 3 40 505 454 500 30 550 48 Comparative Example 1 Inventive Steel 1 15 515 473 480 30 600 20 Comparative Example 2 Inventive Steel 1 40 515 473 480 50 600 30 Comparative Example 3 Inventive Steel 1 40 515 473 650 20 550 48 Comparative Example 4 Invention river 2 40 520 453 400 20 600 30 Comparative Example 5 Invention river 2 40 520 453 500 50 550 48 Comparative Example 6 Invention river 2 15 520 453 500 20 650 30 Comparative Example 7 Invention steel 3 40 505 454 500 50 600 30 Comparative Example 8 Invention steel 3 15 505 454 500 20 600 30 Comparative Example 9 Comparative River 1 40 522 420 500 30 600 30 Comparative Example 10 Comparative River 2 40 522 421 500 30 600 30 Comparative Example 11 Comparative Steel 3 40 495 442 460 20 600 30 Winding temperature upper limit (℃): 600-58C-35Mn-34Cr
Lower Winding Temperature (℃): 555-440C-14Si-26Mn-11Cr

The yield strength, tensile strength, elongation, and yield ratio of the cold rolled steel sheet manufactured under the above conditions were measured, and in addition, the two-phase fraction of the microstructure and the bending workability were measured, and the results are shown in Table 3 below. Here, the biphasic fraction means bainite or martensite structure.

division Yield strength
(MPa) The tensile strength
(MPa) Elongation
(%) Yield ratio 2-phase fraction
(area%) Bending workability Inventory 1 990 1028 14 0.96 0 0R Inventive Example 2 844 887 16 0.95 0 0R Inventory 3 820 898 16 0.91 0 0R Honorable 4 833 922 15 0.90 0 0R Comparative Example 1 489 651 22 0.75 0 0R Comparative Example 2 515 690 18 0.75 0 0R Comparative Example 3 488 667 20 0.73 0 0R Comparative Example 4 492 690 20 0.71 12 0R Comparative Example 5 421 624 22 0.67 18 0R Comparative Example 6 432 603 23 0.72 12 0R Comparative Example 7 578 799 15 0.72 16 1R Comparative Example 8 567 801 16 0.71 15 2R Comparative Example 9 423 644 21 0.66 23 0R Comparative Example 10 381 522 24 0.73 21 0R Comparative Example 11 588 813 16 0.72 29 1R

As can be seen from Tables 1 to 3, in the case of Inventive Examples 1 to 4, which satisfies all of the alloy components, ranges, and manufacturing conditions proposed by the present invention, not only excellent strength is secured but also excellent bending workability. have.

On the other hand, Comparative Examples 9 to 11 that do not satisfy the alloying composition and composition range of the present invention was prepared to satisfy the manufacturing conditions of the present invention, it can be seen that the strength is significantly reduced compared to the invention examples. In particular, in Comparative Example 11, the bending workability is 1R, which shows that the effect is low in terms of workability.

On the other hand, even in the case of satisfying the alloy composition of the present invention it can be seen that in the case of Comparative Examples 1 to 8 that do not satisfy the present production conditions, excellent strength cannot be secured.

Claims (7)

By weight%, C: 0.01-0.12%, Si: 0.1-2.0%, Mn: 0.5-2.5%, P: 0.001-0.10%, S: 0.020% or less, Sol.Al: 0.02-0.30%, N: 0.020 % Or less, Cr: 0.1-1.2%, B: 0.0010-0.0080%, balance Fe and other unavoidable impurities,
In addition, one or two of Ti: 0.003 to 0.08% and Nb: 0.003 to 0.08% are further included.
Satisfy the conditions of the following relations 1 and 2,
Microstructure is ferrite single phase,
Low alloy cold rolled steel with excellent weldability and strength with yield strength of 750MPa or more, tensile strength of 887MPa and yield ratio of 0.9 or more.
[Relation 1]
C + Mn / 6 + Cr / 5 ≤ 0.55
[Relationship 2]
C + Mn / 20 + Si / 30 + 2P + 4S ≤0.27
The method of claim 1,
Low alloy cold rolled steel sheet excellent in weldability and strength that the plating layer is formed on the surface of the cold rolled steel sheet.
By weight%, C: 0.01-0.12%, Si: 0.1-2.0%, Mn: 0.5-2.5%, P: 0.001-0.10%, S: 0.020% or less, Sol.Al: 0.02-0.30%, N: 0.020 % Or less, Cr: 0.1-1.2%, B: 0.0010-0.0080%, balance Fe and other unavoidable impurities,
In addition, one or two of Ti: 0.003 to 0.08% and Nb: 0.003 to 0.08% are further included.
Reheating the slab that satisfies the conditions of the following relations 1 and 2 to 1100 ~ 1300 ℃;
Finishing hot rolling the reheated slab above Ar3 transformation point to obtain a hot rolled steel sheet;
Cooling the hot rolled steel sheet at a rate of 20 to 80 ° C./s;
Winding the cooled hot rolled steel sheet in a range of Coiling Temperature (CT) represented by the following Equation 3;
Cold rolling the hot rolled steel sheet at a rolling reduction rate of 10 to 34% to obtain a cold rolled steel sheet; And
Including the step of annealing the cold-rolled steel sheet at 500 ~ 650 ℃, microstructure is a ferrite single phase, low alloy excellent in weldability and strength to obtain a cold-rolled steel sheet having a yield strength of 50MPa or more, a tensile strength of 887MPa or more and a yield ratio of 0.9 or more Method of manufacturing cold rolled steel sheet.
[Relation 1]
C + Mn / 6 + Cr / 5 ≤ 0.55
[Relationship 2]
C + Mn / 20 + Si / 30 + 2P + 4S ≤0.27
[Relation 3]
555-440C-14Si-26Mn-11Cr ≤ CT ≤ 600-58C-35Mn-34Cr
The method of claim 3,
The microstructure of the hot rolled hot rolled steel sheet is a method of manufacturing a low alloy cold rolled steel sheet having excellent weldability and strength, including ferrite and lower bainite.
The method of claim 3,
The annealing is performed for 3 to 100 hours, the method of manufacturing a low alloy cold rolled steel sheet excellent in weldability and strength.
The method of claim 3,
After the annealing, the method of manufacturing a low-alloy cold rolled steel sheet having excellent weldability and strength, further comprising cooling at a rate of 30 ° C / s or less.
The method of claim 3,
After the annealing step, further comprising the step of forming a plating layer on the cold rolled steel sheet manufacturing method of low alloy cold rolled steel sheet excellent in strength and weldability.